Btrfs: add __init macro to btrfs init functions

Adding __init macro gives kernel a hint that this function is only used
during the initialization phase and its memory resources can be freed up
after.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index 83be8f9..a1a40cf 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -937,7 +937,7 @@ void btrfs_delayed_ref_exit(void)
 	kmem_cache_destroy(btrfs_delayed_extent_op_cachep);
 }
 
-int btrfs_delayed_ref_init(void)
+int __init btrfs_delayed_ref_init(void)
 {
 	btrfs_delayed_ref_head_cachep = kmem_cache_create(
 				"btrfs_delayed_ref_head",